**Effect of Slip on Rotor Parameters**

**4. Effect on Rotor Power Fcator** From rotor impedance, we can write the expression for the power factor of rotor at standstill and also in running condition.

The impedance triangle on standstill condition is shown in the Fig1. From it we can write,

cos Φ

_{2}= Rotor power factor on standstill = R

_{2}/Z_{2 }=R_{2}/√**(**R_{2}^{2}+ X_{2}^{2}**)** The impedance in running condition becomes Z

_{2r }and the corresponding impedance triangle is shown in the Fig.2. From Fig. 2 we can write, cos Φ

_{2}r = Rotor power factor in running condition = R

_{2}/Z_{2r }= R_{2}/√**(**R_{2}^{2}+ (s X_{2})^{2}**)****Key point**: As rotor winding is inductive, the rotor p.f. is always lagging in nature.

Fig. 1 |

Fig. 2 |

**5. Effect on Rotor Current** Let I

_{2 }= Rotor current per phase on standstill condition The magnitude of I

_{2 }depends on magnitude of E_{2 }and impedance Z_{2 }per phase. I

_{2 }= (E_{2 }per phase)/(Z_{2 }per phase) A Substituting expression of Z

_{2 }we get, I

_{2 }= E_{2 }/√**(**R_{2}^{2}+ X_{2}^{2}**)**A The equivalent rotor circuit on standstill is shown in the Fig.3. The Φ

_{2 }is the angle between E_{2 }and I_{2 }which determines rotor p.f. on standstill.Fig. 3 |

In the running condition, Z

_{2 }changes to Z_{2r }while the induced e.m.f. changes to E_{2r}. Hence the magnitude of current in the running condition is also different than on standstill. The equivalent circuit on running condition is shown in the Fig. 4. I

_{2r }= Rotor current per phase in running condition The value of slip depends on speed which inturn depends on load on motor hence X

_{2r}is shown variable in the equivalent circuit. From the equivalent we can write, I

_{2r }= E_{2r}/Z_{2r }= (s E_{2})/√**(**R_{2}^{2}+ (s X_{2})^{2}**)****Key point**: Putting s = 1 in the expression obtained in running condition, the values at standstill can be obtained.

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